This invention relates to a method and apparatus for monitoring a rupture disc for gas permeability.
Vessels which may be subject to a dangerous buildup of pressure are required by the ASME Code to be protected by pressure relieving devices such as safety valves and rupture discs. These devices are designed to allow the vessel to vent upon the obtainment of a vessel pressure which is less than or equal to the vessel's maximum allowable working pressure (MAWP). The vessel pressure at which venting is to begin is commonly referred to as the set pressure. The differential pressure across a rupture disc at rupture is referred to as the burst pressure.
Safety valves and rupture discs are used by placing them in gaseous communication with the vessel's gaseous contents. This can be accomplished by fitting the devices directly to the vessel or on a line leading to the vessel.
While safety valves have many applications, the best engineering practice would not recommend them where there is a chance that the valve could become plugged, as would be the case when the vessel contains solids dissolved in or in admixture with a liquid medium. In these cases, it is preferred to use rupture discs.
Care must be taken with both safety valves and rupture discs when the vessel's contents are corrosive as the operative integrity of these devices can be compromised. Rupture discs are especially useful under corrosive conditions since the disc can be made of a material which is resistant to corrosion and since these discs are fairly easy to change-out when the disc suffers an unacceptable degree of corrosion over time. Safety valves can also be made corrosion resistant material, but should they suffer corrosion, they must be totally replaced at a high cost or must be rebuilt, which rebuilding is not without substantial expense and is time consuming.
Despite the advantages of rupture discs, they are not a panacea. It is not unusual for a rupture disc to develop a pinhole leak due to corrosion, to fatigue or to damage during installation. Such leaks are dangerous as they may compromise the rupture disc's burst pressure rating, and, when the vessel contains a toxic gas, allow for the leakage of the toxic gas into the atmosphere. The art has answered this problem by providing a device which uses a double disc system. In a typical application of this system, two identical rupture discs are mounted in series in a three-piece disc holder so that there is a chamber defined between the two discs. A pressure gauge monitors for any increase in the chamber's pressure. When the rupture disc that is in gaseous communication with the vessel gas develops a leak, the gas from the vessel fills the chamber and causes a rise in chamber pressure. Upon periodic inspection, the gauge reading is seen and the necessary repairs can be effected. While this system is useful, it can be expensive as it can require that the second disc be of a corrosion resistant material if the vessel gas is corrosive. Also, this system can result in an unintentional raising of the set pressure should a pinhole leak form in the disc closest to the vessel. The gas leaked into the chamber can achieve a pressure in the chamber which is equal to the vessel pressure. When this occurs, the set pressure becomes the sum of the vessel pressure and the burst pressure rating of the disc closes to the vessel. Such a sum can far exceed the designed set pressure which was based only upon the burst pressure of the disc.
It is therefore an object of this invention to provide a process and apparatus for safely detecting gaseous permeability in a rupture disc, and for protecting against the escapement of vessel gas into the atmosphere during normal vessel operation.